In the reception and handling of 911 emergency telephone calls, it is important to be able to automatically pinpoint the location of a caller; e.g. an anxious or hysterical caller unable to tell his or her location, or a caller that does not know his/her location and has no visible landmarks that could be used to fully identify such. In calls over ordinary telephone sets directly linked by wire to the Public Switched Telephone Network (hereafter, PSTN), it is possible to trace the number of the telephone from which the call is placed and use that information to locate the caller, since the calling device or unit is associated with a known "building" address from which the caller's location is easily implied or determinable.
However, this is not possible when the respective calling unit is mobile; e.g. a cellular telephone or a "2-way" pager. Mobile units of this kind generally link to the PSTN through a network of geographically dispersed antennas, base stations and switching offices. Although such units have an identity which is signalled during a call, that identity neither implies their physical location nor forms a basis for calculating it. Furthermore, even if the locations of the antennas and distances between them are known, that information per se does not form a basis for determining the location of a unit with which they are currently communicating.
The problem we are concerned with presently is that of obtaining sufficient information about approximate locations of mobile units involved in critical situations (911 calls, emissions from stolen vehicles, etc.), to enable operators on the PSTN to dispatch personnel (rescue workers, firemen, policemen, etc.) with a fair probability of the latter being able to successfully locate respective units. Another problem we are concerned with here is that of providing the foregoing services in a cost-effective manner which places minimal burdens (financial or other) on users of otherwise inexpensive mobile units.
Prior art GPS (Global Positioning System) arrangements are known wherein mobile units involved in public emergency (911) call situations can establish their geographic locations and relay that information to public emergency operators/centers. However, such mobile units require complex radio links to earth satellites, and therefore tend to be too expensive to be used by more than a very small percentage of users involved in such situations.
Other known prior art describes techniques for determining approximate locations of mobile units wherein the determination is made wholly within the existing infrastructure serving such units and require minimal information from the units themselves. However, this art does not concern itself with application of respective techniques to location of callers involved in emergency situations, and their levels of accuracy are considered too low to have useful application to more than a very small percentage of emergency call situations involving mobile units.
Our invention provides a cost-effective technique for determining approximate locations of mobile units, which has practical applications to public emergency calls and other critical situations involving virtually all users of such units, and which is not dependent for practicality upon having the users per se supply information relative to their locations.